Warm air furnaces are frequently used in homes and office buildings to heat intake air received through return ducts and distribute heated air through warm air supply ducts. Such furnaces typically include a circulation fan or blower that directs cold air from the return ducts across a heat exchanger having metal surfaces that act to heat the air to an elevated temperature. An ignition element such as an AC hot surface ignition (HSI) element or direct spark igniter may be provided as part of a gas burner unit for heating the metal surfaces of the heat exchanger. The air heated by the heat exchanger can be discharged into the warm air ducts via the circulation fan or blower, which produces a positive airflow within the ducts. In some designs, a separate inducer fan or blower can be used to remove exhaust gasses resulting from the combustion process through an exhaust vent.
In a conventional warm air furnace system, gas valves are typically used to regulate gas pressure supplied to the burner unit at specific limits established by the manufacturer and/or by industry standard. Such gas valves can be used, for example, to establish an upper gas flow limit to prevent over-combustion or fuel-rich combustion within the appliance, or to establish a lower limit to prevent combustion when the supply of gas is insufficient to permit proper operation of the appliance. In some cases, the gas valve regulates gas pressure independent of the inducer fan. This may permit the inducer fan to be overdriven to overcome a blocked vent or to compensate for pressure drops due to long vent lengths without exceeding the maximum firing rate of the appliance.
In some designs, the gas valve may be used to modulate the gas firing rate within a particular range in order to vary the amount of heating provided by the appliance. Modulation of the gas firing rate may be accomplished, for example, via pneumatic signals received from the inducer fan, or via electrical signals from a controller tasked to control the gas valve. While such techniques are generally capable of modulating the gas firing rate, such modulation is usually accomplished via control signals that are independent from the control of the combustion air flow produced by the inducer fan. In some two-stage furnaces, for example, the gas valve may output gas pressure at two different firing rates based on control signals that are independent of the actual combustion air flow produced by the inducer fan. Since the gas control is usually separate from the combustion air control, the delivery of a constant gas/air mixture to the burner unit may be difficult or infeasible over the entire range of firing rate.
In some systems, supply air temperature and pressure sensors are employed to sense the combustion air flow produced by the inducer fan. Typically, the temperature and pressure sensors will sense the supply air fed to the burner box, which can then be used by the controller to compute mass flow through the combustion side of the furnace. In some designs, a mass flow sensor may also be used in lieu of, the temperature and pressure sensors to compute mass flow.
The addition of these sensors require additional power to operate the furnace, decreasing overall power efficiency. In some cases, the performance of these sensors can degrade over time, causing the furnace to operate at a lower efficiency or to shut-down due to a system fault. The complexity associated with installing these sensors can also increase the level of skill and time required to install and service the furnace system.